Missing return type in array.map

Issue

I got Eslint warning in following TypeScript code:

"Missing return type on function.eslint(@typescript-eslint/explicit-function-return-type)" The warning refers the lambda in map function.

private headerSelectionChanged(event): void {
    const { checked } = event.syntheticEvent.target;
    const newGridData = this.props.gridData.map(item => Object.assign(item, { selected: checked }));
    this.updateGridData(newGridData);
}

What is wrong? How can I fix it?

Solution

The warning shows because your mapping function does not specify a return type. You have to specify the return type like this:

const newGridData = this.props.gridData.map((item): any => Object.assign(item, { selected: checked }));

Keep in mind that Object.assign modifies the target instead of copying it. If you do not want this.props.gridData to be modified, you should swap item and { selected: checked }

const newGridData = this.props.gridData.map((item): any => Object.assign({ selected: checked }, item));

or use the newer object spread operator, if available:

const newGridData = this.props.gridData.map((item): any => ({ ...item, selected: checked }));

Answered By - corschdi